Hierarchical data organization for dense optical flow processing in a computer vision system

ABSTRACT

A computer vision system is provided that includes an image generation device configured to capture consecutive two dimensional (2D) images of a scene, a first memory configured to store the consecutive 2D images, a second memory configured to store a growing window of consecutive rows of a reference image and a growing window of consecutive rows of a current image, wherein the reference image and the current image are a pair of consecutive 2D images stored in the first memory, a third memory configured to store a sliding window of pixels fetched from the growing window of the reference image, wherein the pixels in the sliding window are stored in tiles, and a dense optical flow engine (DOFE) configured to determine a dense optical flow map for the pair of consecutive 2D images, wherein the DOFE uses the sliding window as a search window for pixel correspondence searches.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 15/638,142, filed Jun. 29, 2017, which is incorporated by reference herein in its entirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

Embodiments of the present disclosure generally relate to computer vision systems, and more specifically relate to dense optical flow processing in such systems.

Description of the Related Art

Advanced driver assistance systems, or ADAS, are becoming increasingly commonplace in automobiles. These systems provide the driver with the information about the surrounding and potentially automatically intervene with the process of driving to prevent accidents and road fatalities. Three dimensional (3D) scene understanding using image sensors is an elementary step in building this scene understanding. Scene motion understanding using optical flow is a common technique that provides accurate motion estimation. For determining optical flow, one camera is employed to capture temporal sequences of images (video) of the same scene. Pairs of consecutive images are processed to determine the displacement of pixels between consecutive images. The displacement or flow vector information for the pixels can be used to analyze the motion of objects/volume corresponding to the pixels in the captured scene.

One approach to optical flow that may be used in embedded safety systems is sparse optical flow. Sparse optical flow is a feature-based approach in which only the flow of selected pixels, e.g., features such as image edges, corners, etc., is identified across consecutive images. A more accurate approach to optical flow is dense optical flow in which the flow of all pixels in consecutive images is identified. Using dense optical flow in embedded systems is challenging given the real time processing requirements of such systems as current approaches to dense optical flow processing may be computation intensive and require significant data bandwidth.

SUMMARY

Embodiments of the present disclosure relate to methods and apparatus for hierarchal data organization for dense optical flow processing in a computer vision system. In one aspect, a computer vision system is provided that includes an image generation device configured to capture consecutive two dimensional (2D) images of a scene, a first memory configured to store the consecutive 2D images, a second memory configured to store a growing window of consecutive rows of a reference image fetched from the first memory and a growing window of consecutive rows of a current image fetched from the first memory, wherein the reference image and the current image are a pair of consecutive 2D images, a third memory configured to store a sliding window of pixels fetched from the growing window of consecutive rows of the reference image, wherein the pixels in the sliding window are stored in tiles, and a dense optical flow engine (DOFE) configured to determine a dense optical flow map for the pair of consecutive 2D images, wherein the DOFE uses the sliding window as a search window for pixel correspondence searches.

In one aspect, a method of operating a computer vision system to determine a dense optical flow map for two consecutive two dimensional (2D) images of a scene is provided that includes storing the two consecutive 2D images in a first memory of the computer vision system, maintaining a growing window of consecutive rows of a reference image of the two consecutive 2D images and a growing window of consecutive rows of a current image of the two consecutive 2D images in a second memory of the computer vision system, wherein the consecutive rows of the reference image and the consecutive rows of the current image are fetched from the first memory, maintaining a sliding window of pixels of the reference image in a third memory, wherein the pixels are fetched from the growing window of consecutive rows of the reference image in the second memory, and wherein the pixels are stored in tiles in the sliding window, and determining the dense optical flow map for the two consecutive 2D images by a dense optical flow engine (DOFE) of the computer vision system, wherein the DOFE uses the sliding window as a search window for pixel correspondence searches.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments will now be described, by way of example only, and with reference to the accompanying drawings:

FIG. 1 is a high level block diagram of an example multiprocessor system-on-a-chip (SOC);

FIG. 2 is high level block diagram of an example optical flow accelerator of the SOC of FIG. 1;

FIG. 3 illustrates the processing steps of the dense optical flow engine (DOFE) of the optical flow accelerator of FIG. 2 to generate a dense optical flow vector map;

FIG. 4 shows example spatial predictors for the predictor search of the DOFE;

FIG. 5 is a high level block diagram of an example architecture for the DOFE;

FIG. 6 shows an example data hierarchy to support the optical flow processing of the DOFE;

FIG. 7 shows an example sliding search window used in the optical flow processing of the DOFE;

FIG. 8 is an example illustrating a tiled organization of the sliding search window;

FIG. 9 is an example illustrating best and worst case memory accesses using the tiled organization; and

FIG. 10 is a flow diagram of a method for determining a dense optical flow map.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

Specific embodiments of the disclosure will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency.

A moving object in the real world has a three dimensional (3D) motion. A camera capturing a moving object captures a two dimensional (2D) projection of the actual 3D motion. The 2D motion of a moving object can be determined by observing consecutive captured images of the object. The 2D motion of an image pixel from image to image is referred to as the optical flow of the pixel and is defined as the displacement vector of the pixel between two consecutive images. For dense optical flow, the optical flow of each pixel is determined.

Determination of the optical flow of a pixel is fundamentally a “best correspondence” finding problem. That is, given two consecutive images, the problem is to find the best matching position of a pixel in the older image in the newer image. In general, the search for the best matching pixel is performed within a predetermined region, referred to as the search range, in the newer image. A predictor based search strategy may be used to avoid searching for correspondence at every pixel in the search range.

Embodiments of the disclosure provide for dense optical flow processing in an embedded computer vision system that meets real time performance requirements. In some embodiments, a hardware accelerator for dense optical flow map calculation is provided. The hardware accelerator includes novel features that improve the performance of dense optical flow computation such as a paxel based search for matching pixels that reduces search time, a hierarchical data organization with tiling to manage data bandwidth, and/or advanced predictor evaluation that avoids refetching of data.

FIG. 1 is a high level block diagram of an example multiprocessor system-on-a-chip (SOC) 100 configured to support computer vision processing in a camera-based ADAS. The SOC 100 includes dual general purpose processors (GPP) 102, dual digital signal processors (DSP) 104, a vision processor 106, and an optical flow accelerator 112 coupled via a high speed interconnect 122. As is explained in more detail herein, the optical flow accelerator 112 is configured to perform dense optical flow processing on incoming pairs of camera images. The vision processor 106 is a vector processor tuned for computer vision processing such as gradient computation, orientation binning, histogram normalization, etc. The GPP 102 hosts the operating system and provides overall control of the operation of the SOC 100 including scheduling of the processing performed by the optical flow accelerator 112. The DSP 104 provides support for computer vision processing such as object detection and classification, which may use the output of the optical flow accelerator 112.

The SOC 100 further includes a direct memory access (DMA) component 108, a camera capture component 110 that may be coupled to an external camera 124, a display management component 114, on-chip random access (RAM) memory 116, e.g., a computer readable medium, and various input/output (I/O) peripherals 120 all coupled to the processors and the optical flow accelerator 112 via the interconnect 122. The camera 124 may be coupled to the SOC 100 via an analog connection, an Ethernet cable, a low-voltage differential signaling (LVDS) cable, or other suitable wired or wireless connection. In addition, the SOC 100 includes a safety component 118 that includes safety related functionality to enable compliance with automotive safety requirements. Such functionality may include support for CRC (cyclic redundancy check) of data, clock comparator for drift detection, error signaling, windowed watch-dog timer, and self testing of the SOC for damage and failures.

FIG. 2 is a high level block diagram of an example optical flow accelerator 112. The optical flow accelerator 112 includes a configuration interconnect 206, a hardware thread scheduler (HTS) 204, a dense optical flow engine (DOFE) 202, memory mapped registers (MMR) 208, a shared memory interconnect 210, and shared memory 212.

The configuration interconnect 206 provides an interface for configuring the parameters of the optical flow accelerator 112. The configuration interconnect 206 also provides an interface for observing the status to the processing performed by the optical flow accelerator 112.

The HTS 204 implements thread management and control triggering of processing threads in the DOFE 202. The HTS 204 also manages message transfer and control between the optical flow accelerator 112 and other components in the SOC 100, e.g., the DSP 104.

The shared memory 212 stores input and output data for the dense optical flow engine 202. The shared memory 212 includes four banks of static random access memory. The shared memory interconnect 210 is a crossbar with pipelined command and response handling. The DMA 108 is connected to the shared memory interconnect 210 and is used to move data for processing by the DOFE 202 into the shared memory and to move the optical flow data produced by the DOFE 202 out of the optical flow accelerator 112 for consumption by other components on the SOC 100.

The MMR 208 is configured to store various control and configuration parameters for the DOFE 202. Parameters may include buffer sizes and addresses in the shared memory 212, horizontal and vertical search ranges, confidence score configuration values, etc.

The DOFE 202 is configured to find the optical flow vector map between all pixels of a pair of input images, referred to as the reference image and the current image herein. The current image is the image received at time t−1 and the reference image is the image received at time t. The dense optical flow approach implemented by the DOFE 202 is based on motion/flow vector predictors and uses a coarse-to-fine correspondence search strategy leveraging image pyramids of the reference image and the current image.

The coarse-to-fine correspondence search is based on the fact that a large displacement of a pixel between two consecutive images can be reduced to a smaller displacement at lower spatial resolution, thereby reducing the complexity of the correspondence search problem. For this search approach, a successively scaled down image structure is created from each input image. For example, a Gaussian image pyramid for an image may be created by filtering an input image using a 5×5 Gaussian low pass filter and then down sampling the filtered image by a factor of two in the horizontal and vertical directions. The resulting image is then subjected to the same process, and the cycle is repeated until the desired number of pyramid levels, e.g., six levels, is generated. Each cycle of this process results in a smaller image with increased smoothing and decreased spatial sampling density, i.e., decreased image resolution.

In the resulting pyramid, the original image may be referred to as the base pyramid level or level 0 and the smaller resolution images may be referred to as higher pyramid levels, e.g., levels 1 to 5, assuming there are five higher levels. Note that in the example Gaussian image pyramid, each pyramid level is ¼ the size of the next lower level. Thus a pixel at a particular pyramid level corresponds to a 2×2 block of four pixels at the next lower pyramid level. Also a distance measured at a higher pyramid level is ½ of the distance at the next lower pyramid level.

FIG. 3 illustrates the processing steps of the DOFE 202 to generate a dense optical flow vector map. This figure assumes that the image pyramids have five upper levels generated as described above. One of ordinary skill in the art will understand that more or fewer levels may be used. Each of the pyramid levels is processed in turn to generate an optical flow vector for each pixel in the level image beginning with the highest pyramid level. At each upper pyramid level, a predictor based search strategy 300 is applied to find an initial match which is then further refined with a hierarchical step search strategy 302 resulting in an integer pixel accurate flow vector for each pixel. A median filter 304 is applied to the flow vectors to remove impulsive estimation noise. The filtered flow vectors are scaled up 306 by two and used as initial predictors at the next lower pyramid level.

At the base pyramid level, the predictor based search strategy 308 is applied to original images to find initial matches which are then further refined with the hierarchical step search strategy 310 resulting in an estimated integer pixel accurate flow vector for each pixel. After determining the integer pixel flow vectors, the pixel correspondence is further refined 312 to a sub-pixel level accuracy. A median filter 314 is applied to the refined flow vectors to remove impulsive estimation noise. Any suitable median filter may be used. A confidence map 316 is generated by computing a quantitative estimate of correctness, referred to as the confidence score, for each final estimated flow vector. The final estimated flow vectors and the confidence map are then packed together to form a dense optical flow map which is stored in external memory. Any suitable technique for computing confidence scores may be used.

The predictor search 300, 308 and the step search 302, 310 at each pyramid level search at each pyramid level find the best matching pixel in the reference image for each pixel in the current image. For a pixel in the current image, the search for the best matching pixel in the reference frame is restricted to a search window in the current frame. The best match is determined based a matching cost computed for the current pixel and each candidate matching pixel in the reference image. Prior to computing the matching cost, a census transform is applied to the current pixel value and the candidate matching reference pixel value to reduce error from illumination variations and a census signature for each pixel is computed based on the respective transformed value. The matching cost is then measured based on the Hamming distance between the census signatures of the pixels. Any suitable census transform and approach to computing the census signatures and Hamming distance may be used.

The predictor search 300, 308 uses predictors to prune the full search window to avoid searching for correspondence at each pixel in the search window. Such a search basically assumes spatial and temporal locality and smoothness of motion, i.e., that pixels in a small neighborhood are highly likely to have similar motion and that motion changes smoothly both spatially and temporally. Further, rather than searching for each pixel in the current frame, the search is performed for non-overlapping blocks of pixels, which is may be referred to as paxels herein. For example purposes, a paxel is assumed to be a 2×2 block of pixels. Other suitable block sizes may be used.

In the predictor search 300, 308, every 2×2 block of pixels at any pyramid level is assumed to be predictable by a set of flow vector values, which may be referred to as flow vector predictors or predictors, which are selected from the actual/estimated flow vectors of spatial or temporal neighbors of the block. The predictors are defined with respect to the top left pixel of the 2×2 pixel block, which may be referred to as the anchor pixel herein. Any suitable type and/or number of predictors may be used. In some embodiments, five spatial predictors and a single temporal predictor are used. The five spatial predictors are illustrated in FIG. 4. The spatial predictors are the flow vector of the pixel at the top left (TL) of the anchor pixel, the flow vector of the pixel at the top (T) of the anchor pixel, the flow vector of the pixel at the top right (TR) of the anchor pixel, the flow vector of the pixel to the immediate left of the anchor pixel as estimated in the previous pyramid level, and the flow vector of the pixel in the same position as the anchor pixel as estimated in the previous pyramid level. The temporal predictor is the flow vector of the pixel in the same position as the anchor pixel as estimated in the previous image.

In some embodiments, an additional spatial predictor is used, the flow vector of a pixel that is n pixels to the left of the anchor pixel in the same row. The value of n depends on the pipeline depth of the DOFE 202. This predictor may be referred to as the delayed left predictor.

The spatial predictors are used at all pyramid levels and the temporal predictor is used in the base pyramid level. The predictor that results in a matching candidate with the minimum matching cost is selected as the winning predictor for the paxel. The step search 302, 310 is then applied around the winning predictor from the predictor search 300, 308 to further refine the matching position. The step search 302, 310 is intended to refine the best matching position from the winning predictor in a limited two dimensional (2D) area around the winning predictor position. The step search 302, 310 is performed for each pixel in the paxel to determine a flow vector for each pixel.

In some embodiments, a hierarchical step search technique referred to as the 3-1 step search is used. Initially, eight pixel positions around the winning predictor position are searched. The eight pixel positions are selected such that the positions are three pixels away from the winning predictor position in each of eight directions, i.e., north, northeast, east, southeast, south, southwest, west, and northwest. The best match position from this search is further refined by choosing eight similarly positioned pixels at a one pixel distance from the best match position. The best match position from this one pixel search is selected as the final integer pixel accurate flow vector.

FIG. 5 is a high level block diagram of an example architecture for the DOFE 202 of FIG. 2. A census transform component 506, a predictor search engine (PSE) 504 and a step search engine (SSE) 502 are coupled in series and are configured to perform the previously described census transform, predictor search, and step search on current and reference pixel data stored in the search range buffer 508. A sub-pixel refinement component 518 is configured to perform the previously described sub-pixel refinement on the best match positions resulting from the step search performed at the base pyramid level. A median filter component 514 is configured to perform the previously described median filtering and a confidence score component 516 is configured to generate confidence scores for the selected best match positions.

The output packing component 520 is configured to pack a stream of flow vectors from the median filter component 514 and a stream of corresponding confidence scores from the confidence score component 516 to produce a flow vector map that is stored in the output buffer 522. The shared memory interface 512 is configured to read the flow vector map from the output buffer 522 and store the map in the shared memory 512 via the shared memory interconnect 210. The flow vector map may then be transferred to external memory from the shared memory 512, e.g., the on-chip RAM 116 of FIG. 1, by the DMA 108.

To optimize access bandwidth to data needed for the correspondence searches, a hierarchical data organization is used. As illustrated in FIG. 6, in some embodiments, the data hierarchy includes three levels of memory in which the amount of storage progressively decreases from level 3 (L3) to level 1 (L1) while the access bandwidth progressively increases. The L3 memory may be external to the optical flow accelerator 112, e.g., the on-chip RAM 116, while the L2 memory and the L1 memory are included in the optical flow accelerator 112 as the shared memory 212 and the search range buffer 508, respectively.

In this hierarchy, the L3 memory stores the reference and current images, the L2 memory stores a subset of concurrent pixel rows of each of the images, and the L1 memory stores a search window extracted from the reference image rows in the L2 memory. Further, the subsets of the reference image and the current image in the L2 memory may be maintained according to a growing window approach in which a circular buffer of consecutive image rows read from the L3 memory is maintained for each image in parallel with the optical flow processing. The number of rows in each of the circular buffers may be any suitable number. In some embodiments, the minimum number of rows in a growing window may be based on the number of pixels needed for the census transforms and the number of pixels needed for computing the Hamming distance as well as the vertical search range. In some embodiments, image rows are fetched from the L3 memory in paxel rows, e.g., if a paxel is 2×2, two rows are fetched.

As depicted in FIG. 7, the search window in the L1 memory, e.g., the search range buffer 508, may be maintained according to a sliding window approach in which a circular buffer of consecutive columns of pixel tiles read from the reference image growing window is maintained in parallel with the optical flow processing. As paxel processing moves right in raster scan order, new columns of pixel tiles are added while the same number of pixel tile columns is discarded. The size of the sliding window may be based on the search range for the correspondence matching.

A tiled organization is used in the search window of the L1 memory to minimize the number of memory accesses needed to perform the correspondence search. A pixel region of a size dictated by the census transform and the matching cost computation used is needed for a candidate pixel position in the reference image. Because motion is unconstrained, the flow vectors and predictors can point to any pixel position within the search range. So the pixel region for the cost computation can be centered at any pixel. If the pixels are in raster scan order, access to all the pixels in the pixel region may require multiple accesses. Arranging the reference image pixels in pixel tiles reduces the number of accesses needed.

Further, the tiles are “striped” across multiple memory banks in the L1 memory. Any suitable number of memory banks may be used. Each memory bank is sized to store multiples of whole tiles and a tile is stored in a memory bank such that it can be accessed in a single cycle. The particular arrangement of the tiles across the memory banks may depend, for example, on the number of memory banks available and the size of the search window.

Consider the example of FIG. 8. In this example, a 4×4 tile and four banks of memory are assumed. One of ordinary skill in the art will understand embodiments for other tiles sizes and/or other memory organizations. A small example image 802 is shown in which the raster scan ordered pixels are logically dividing into 4×4 tiles. An example L1 memory 804 is also shown. With this pixel organization, a pixel region can be decomposed in terms of 4×4 tiles and read by a limited number of accesses. For example, as illustrated in FIG. 9, a 14×14 2D region is contained in sixteen (best case) or twenty-five (worst case) 4×4 pixel tiles based on the position of the search vector, thus requiring sixteen (best case) or twenty-five (worst case) accesses to fetch the needed pixel region. In another example, there may sixteen banks of L1 memory in which vertically adjacent 4×4 tiles are placed in different banks.

Referring back to FIG. 5, the shared memory interface 512 is further configured to read pixel data for the reference frame from the growing window in the shared memory 212 via the shared memory interconnect 210. The pixel data is provided to the line to block conversion component 510 for processing and storage in the search range buffer 508. The line to block conversion component 510 is configured to convert the pixel data read from the shared memory 212 into pixel tiles for storage in the search range buffer 508. Any suitable pixel data size and pixel tile size may be used.

In some embodiments, in addition to performing the step search, the SSE 502 also performs advanced predictor evaluation for some of the predictors used by the PSE 504. That is, after determining the winning flow vector for each pixel of the current paxel, the SSE 502 evaluates the matching cost for any of these flow vectors that will be used as predictors for the next row of paxels. The flow vectors and a matching cost for the flow vectors are stored, e.g., in a first-in-first-out buffer, for use by the PSE 504. In some embodiments, a single matching cost that is the minimum of the matching costs of the flow vectors is stored.

For example, assuming a 2×2 paxel and the previously mentioned example predictors, the flow vectors of the bottom two pixels of a current paxel may be top and top right predictors for the paxel immediately below the current paxel. Thus, the SSE 502 evaluates the matching cost for these two flow vectors with respect to the pixel that will be the anchor pixel of the paxel immediately below. Further, the bottom right pixel of the current paxel may be a top left predictor for the paxel below and to the right of the current paxel. Thus, the SSE 502 evaluates the matching cost for that flow vector with respect to the pixel that will be the anchor pixel of the paxel below and to the right.

FIG. 10 is a flow diagram of a method for determining a dense optical flow map in a computer vision system such as the computer vision system of FIGS. 1, 2, and 5. To determine the dense optical flow map, the reference image and the current image are stored 1000 in L3 memory. Further, as the processing is performed to determine the dense optical flow map, a growing window of consecutive rows of the reference image is maintained 1002 in L2 memory and a growing window of consecutive rows of the current image is also maintained 1004 in L2 memory. A sliding window of pixel tiles formed from pixels read from the reference image growing window is also maintained in L1 memory. The dense optical flow map is determined 1008 using the sliding window as the search window for pixel correspondence searches.

OTHER EMBODIMENTS

While the disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the disclosure as disclosed herein.

For example, embodiments have been described herein in reference to ADAS. One of ordinary skill in the art will understand embodiments for computer vision applications other than ADAS, such as, for example, industrial applications, robotics, and drones.

In another example, embodiments have been described herein in which the anchor pixel of a paxel is assumed to be the top left pixel. One of ordinary skill in the art will understand embodiments in which the anchor pixel may be another pixel in the paxel.

In another example, embodiments have been described herein in which the current and reference images are captured by a camera. One of ordinary skill in the art will understand embodiments in which another image generation device may be used to generate the current and reference images. For example, a graphics processor may generate consecutive synthetic images of a scene, e.g., for gaming or virtual reality applications.

In another example, embodiments have been described herein in the context of processing sequential 2D pixel images. One of ordinary skill in the art will understand embodiments in which the input is a pair of current and reference 2D maps of data points and the output is the displacement of each data point between the current 2D map and the reference 2D map. Such 2D maps of data points may include non-sequential camera images in which the pixels are the data points, heat maps in which temperatures are the data points, pressure maps with in which pressure measurements are the data points, occupancy maps in which occupancies are the data points, etc.

Although method steps may be presented and described herein in a sequential fashion, one or more of the steps shown in the figures and described herein may be performed concurrently, may be combined, and/or may be performed in a different order than the order shown in the figures and/or described herein. Accordingly, embodiments should not be considered limited to the specific ordering of steps shown in the figures and/or described herein.

Certain terms are used throughout the description and the claims to refer to particular system components. As one skilled in the art will appreciate, components in systems may be referred to by different names and/or may be combined in ways not shown herein without departing from the described functionality. This document does not intend to distinguish between components that differ in name but not function. In the description and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” Also, the term “couple” and derivatives thereof are intended to mean an indirect, direct, optical, and/or wireless electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, through an indirect electrical connection via other devices and connections, through an optical electrical connection, and/or through a wireless electrical connection, for example.

It is therefore contemplated that the appended claims will cover any such modifications of the embodiments as fall within the true scope of the disclosure. 

What is claimed is:
 1. A method comprising: receiving a first image that includes a first set of pixels and a second image that includes a second set of pixels; dividing the first set of pixels into subsets of pixels; for each of the subsets of pixels: selecting an anchor pixel in the respective subset of pixels; for each of a set of predictors: applying the respective predictor to the anchor pixel to determine a predicted position in the second set of pixels; and determining a matching cost for the predicted position; and determining a closest matching region in the second set of pixels for the respective subset of pixels based on the matching costs.
 2. The method of claim 1, wherein the set of predictors for the anchor pixel includes a spatial predictor for a pixel of the first set of pixels that is proximate to the anchor pixel and that is not in the respective subset of pixels of the anchor pixel.
 3. The method of claim 1, wherein the set of predictors for the anchor pixel includes: a first spatial predictor for a first pixel that is directly adjacent to the anchor pixel; a second spatial predictor for a second pixel that is directly diagonal to the anchor pixel; and a third spatial predictor for a third pixel that is directly diagonal to the anchor pixel.
 4. The method of claim 1, wherein the set of predictors for the anchor pixel includes a spatial predictor for a pixel of a third image that is down sampled from the first image.
 5. The method of claim 1, wherein the set of predictors for the anchor pixel includes a temporal predictor for a pixel of the second set of pixels in a same position as the anchor pixel.
 6. The method of claim 1, wherein the determining of the closest matching region for the respective subset of pixels includes determining a closest matching predictor of the set of predictors based on the matching costs of the set of predictors.
 7. The method of claim 6, wherein: the determining of the closest matching region for the respective subset of pixels includes applying the closest matching predictor to the respective subset of pixels to determine the closest matching region; and the method further comprises performing a step search around the closest matching region to determine a flow vector for each pixel in the respective subset of pixels.
 8. The method of claim 7 further comprising providing the flow vectors for determining optical flow of a third image, wherein the first image is down sampled from the third image.
 9. The method of claim 7 further comprising: filtering the flow vectors to produce filtered flow vectors; scaling the filtered flow vectors to produce scaled flow vectors; and providing the scaled flow vectors for determining optical flow of a third image, wherein the first image is down sampled from the third image.
 10. The method of claim 1, wherein each of the subsets of pixels is a 2×2 non-overlapping set of pixels.
 11. The method of claim 1, wherein the first image is subsequent to the second image in time.
 12. A circuit device comprising: a predictor search engine configured to: receive a first image and a second image; divide the first image into subsets of pixels; for each of the subsets of pixels: select an anchor pixel in the respective subset of pixels; for each of a set of predictors: apply the respective predictor to the anchor pixel to determine a predicted position in the second image; and determine a matching cost for the predicted position; and determine a closest matching predictor in the second image for the respective subset of pixels based on the matching costs; and a step search engine coupled to the predictor search engine and configured to, for each of the subsets of pixels: apply the closest matching predictor to the respective subset of pixels to determine a predictor position; and perform a step search around the predictor position to determine an optical flow vector for each pixel in the respective subset of pixels.
 13. The circuit device of claim 12 further comprising: a median filter component coupled to the step search engine and configured to apply a median filter to the optical flow vectors.
 14. The circuit device of claim 12, wherein the circuit device is configured to: scale the optical flow vectors to produce scaled optical flow vectors; and provide the scaled optical flow vectors to the predictor search engine for use in a set of predictors for a third image.
 15. The circuit device of claim 14, wherein the first image is down sampled from the third image.
 16. The circuit device of claim 12, wherein the set of predictors for the anchor pixel includes a spatial predictor for a pixel of the first image that is proximate to the anchor pixel and that is not in the respective subset of pixels of the anchor pixel.
 17. The circuit device of claim 12, wherein the set of predictors for the anchor pixel includes: a first spatial predictor for a first pixel that is directly adjacent to the anchor pixel; a second spatial predictor for a second pixel that is directly diagonal to the anchor pixel; and a third spatial predictor for a third pixel that is directly diagonal to the anchor pixel.
 18. The circuit device of claim 12, wherein the set of predictors for the anchor pixel includes a spatial predictor for a pixel of a third image that is down sampled from the first image.
 19. The circuit device of claim 12, wherein the set of predictors for the anchor pixel includes a temporal predictor for a pixel of the second image in a same position as the anchor pixel.
 20. The circuit device of claim 12, wherein each of the subsets of pixels is a 2×2 non-overlapping set of pixels. 